Wafer polishing slurry and chemical mechanical polishing (CMP) method using the same

ABSTRACT

A chemical mechanical polishing slurry includes an additive of a quaternary ammonium compound having a form of {N-(R 1 R 2 R 3 R 4 )} + X − , in which R 1 , R 2 , R 3 , and R 4  are radicals, and X − is an anion derivative including halogen elements. Preferably, the quaternary ammonium compound is one of [(CH 3 ) 3 NCH 2 CH 2 OH]Cl, [(CH 3 ) 3 NCH 2 CH 2 OH]l, [(CH 3 ) 3 NCH 2 CH 2 0H]Br, [(CH 3 ) 3 NCH 2 CH 2 H]CO 3 , and mixtures thereof. The slurry may further include a pH control agent formed of a base such as KOH, NH 4 OH, and (CH 3 ) 4 NOH, and an acid such as HCI, H 2 SO 4 , H 3 PO 4 , and HNO 3 . Also, the pH control agent can include [(CH 3 ) 3 NCH 2 CH 2 OH]OH. The slurry may further include a surfactant such as cetyldimethyl ammonium bromide, cetyldimethyl ammonium bromide, polyethylene oxide, polyethylene alcohol or polyethylene glycol.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention generally relates to semiconductor devices,and more particularly, the present invention relates to a semiconductorwafer polishing slurry and to a chemical mechanical polishing (CMP)method using such a slurry.

[0003] 2. Description of the Related Art

[0004] Chemical mechanical polishing (CMP) planarization methods arewidely employed in a variety of processes used to manufacturesemiconductor devices. CMP can be used to planarize a single materiallayer, such as CMP of an interdielectric layer or metal interdielectriclayer, or to polish materials having two or more layers, such as CMP inshallow trench isolation (STI) process. In the case of polishingmaterials having two or more layers (such a layer to be polished and apolishing stop layer), it is generally necessary that the CMP processexhibit high selectivity between the layers.

[0005] For example, when an insulating material such as a silicon oxidelayer (SiO₂ layer) is planarized, generally a silicon nitride layer(Si₃N₄ layer) is used as a polishing stop layer. When present CMPtechniques are employed in which the chief ingredient of the slurry issilica, the removal selectivity of SiO₂:Si₃N₄ is known to be about 4:1in a blanket wafer. This relatively low removal selectivity reduces theprocess margins and necessitates very fine CMP process control.

[0006] There is thus a desire for a CMP slurry that exhibits an improvedremoval selectivity between a layer to be polished and a silicon nitridepolishing stop layer.

SUMMARY OF THE INVENTION

[0007] An objective of the present invention is to provide a CMP slurrythat exhibits an improved removal selectivity between a layer to bepolished and a silicon nitride polishing stop layer, and to provide apolishing method using such a slurry.

[0008] According to one aspect of the present invention, a CMP slurryincludes quaternary ammonium compound which is a compound having theform of {N-(R₁R₂R₃R₄)}⁺X⁻, where R₁, R₂, R₃, and R₄ are radicals andX⁻is an anion derivative.

[0009] R₁, R₂, R₃, and R₄ of the quaternary ammonium compound can be analkyl group or an alkanol group, and R₁, R₂, and R₃ are generally amethyl group and R₄ is an ethanol group. Preferably, the quaternaryammonium compound can be [(CH₃)₃NCH₂CH₂OH]Cl, [(CH₃)₃NCH₂CH₂OH]l,[(CH₃)₃NCH₂CH₂OH]Br, [(CH₃)₃NCH₂CH₂OH]CO₃, or mixtures thereof.

[0010] The slurry can further include a pH control agent formed of abase such as KOH, NH₄OH or (CH₃)₄NOH, or an acid such as HCl, H₂SO₄,H₃PO₄ or HNO3. Also, a pH control agent can include [(CH₃)₃NCH₂CH₂OH]OH.

[0011] The slurry can further include a surfactant such as cetyldimethylammonium bromide, cetyidimethyl ammonium bromide, polyethylene oxide,polyethylene alcohol, or polyethylene glycol.

[0012] According to another aspect of the present invention, a polishingmethod includes providing a semiconductor wafer having a silicon oxidelayer and a silicon nitride stop layer, and chemical mechanicalpolishing the silicon oxide layer and the silicon nitride stop layerusing above-describe slurry.

BRIEF DESCRIPTION OF THE DRAWINGS

[0013] The above and other aspects and advantages of the presentinvention will become more readily apparent from the detaileddescription that follows, with reference to the accompanying drawings,in which:

[0014]FIG. 1 is a figure illustrating [(CH₃)₃NCH₂CH₂OH]Cl added to aslurry according to a preferred embodiment of the present invention;

[0015]FIG. 2 is a graph illustrating the results of measurements whichwere performed to demonstrate the change in removal selectivityresulting from the addition of the [(CH₃)₃NCH₂CH₂OH]Cl to the slurryaccording to a preferred embodiment of the present invention; and

[0016]FIG. 3 is a graph illustrating the results of measurements whichwere performed to demonstrate the change in removal selectivityresulting from the addition of a pH control agent to the slurryaccording to a preferred embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

[0017] Hereinafter, preferred embodiments of the present invention willbe described in greater detail with reference to the appended drawings.The preferred embodiments of the present invention may be modified invarious ways, however, and the scope of the invention is not limited tothe preferred embodiments. Rather, the preferred embodiments of thepresent invention are presented to explain the present invention morecompletely to those skilled in the art.

[0018] According to a preferred embodiment of the present invention, aslurry for polishing a wafer includes a solvent, such as deionizedwater, and polishing particles suspended in the solvent. The polishingparticles can be formed of a metal oxide such as silica, alumina,titania, zirconia, germania, or ceria. The polishing particles compriseabout 1-30 wt% of the entire slurry. Also, an additional agent such asan oxidizer can be further included in the slurry as occasion demands.

[0019] According to a preferred embodiment of the present invention,quaternary ammonium compound having a form of {N-(R₁R₂R₃R₄)}^(+X) ⁻isadded to the slurry. Here, R₁, R₂, R₃, and R₄ are radicals, for example,alkyl groups or alkanol groups, and X⁻is an anion derivative includinghalogen elements. For example, R₁, R₂, and R₃ can be methyl groups andR₄ can be an ethanol group.

[0020] A preferred example of the quaternary ammonium compound can be[(CH₃)₃NCH₂CH₂OH]Cl, [(CH₃)₃NCH₂CH₂OH]I, or [(CH₃)₃NCH₂CH₂OH]Br. Thesecompounds can be called choline-halogen compounds, and a preferredembodiment can be [(CH₃)₃NCH₂CH₂OH]Cl as shown in FIG. 1. The compounds[(CH₃)₃NCH₂CH₂OH]l and [(CH₃)₃NCH₂CH₂OH]Br have a structure in which land Br, respectively, are substituted for the Cl of the choline-halogencompound shown in FIG. 1. That is, choline-derivative compounds combinedwith a derivative including halogens, can be preferably used as thecholine-halogen compound.

[0021] Also, a choline derivative compound such as a well known[(CH₃)₃NCH₂CH₂OH]CO₃, can be used. The choline-derivative compoundincludes a choline base (C₅H₁₅NO₂), choline dihydrogen citrate(C,₁H₂₁NO₈), choline bitartrate (C₉H₁₉NO₇), tricholine citrate(C₂₁H₄₇N₃0₁₀), choline ascorbate (C₁₁H₂,NO₇), choline borate(C₅H₁₆BNO₄), choline theophylinate (C₁₁H₂₁H₆O₃), choline gluconate(C₁₁H₂₅NO₈), acetylcholine chloride (C₇H₁₆CINO₂), acetylcholine bromide(C₇H₁₆BrNO₂), or metacholine chloride (C₈H₁₆CINO₂).

[0022] These choline-derivative compounds, which are preferablyderivative compounds including choline-halogen elements, of about 0.1-5wt%, can be added to the slurry, preferably in an amount of about 0.3-3wt%. These choline-derivative compounds are added to the slurry anddissociated to form a choline cation and a derivative anion. Forexample, the choline chloride of FIG. 1 can form a choline cation,[(CH₃)₃NCH₂CH₂OH]⁺and a halogen anion Cl⁻.

[0023] Choline cations are attracted to negative electric charge siteswhich exist on the surface of the silicon nitride layer, and enable astrong attraction such as a hydrogen bond, or enable static electricitybetween the cations and the negative electric charge sites on thesurface. More particularly, structures such as ≡N, =N-H, and -NH₂ can bemicroscopically formed on the silicon nitride layer which is formed overthe semiconductor wafer, and in the base atmosphere, some =N-H, and -NH₂groups are ionized and have a negative electric charge. This negativeelectric charge can form an attraction such as a hydrogen bond or staticelectricity with a choline cation. In fact, since a choline derivativeof an ethanol group has an excellent polarity, an attraction such as ahydrogen bond or the static electricity with the sites having thenegative electric charge can be effectively formed.

[0024] Accordingly, the surface of the silicon nitride layer can bepassivated by the choline cation attracted by the hydrogen bond.Therefore, if CMP is performed on the surface of a silicon nitride layerusing the slurry suggested in the preferred embodiments of the presentinvention, the silicon nitride layer is polished to a lesser extent thanthe silicon oxide layer. Since the structures such as ≡N, =N-H, and -NH₂cannot be formed on the surface of the silicon oxide layer, the removalrate of the silicon nitride layer is lower than that of the siliconoxide layer in which the passivation does not occur. Thus, the removalselectivity of the silicon oxide layer on the silicon nitride layer canbe effectively increased.

[0025] Effects embodied by the preferred embodiments of the presentinvention will be described below with reference to specific examples.

EXAMPLE 1

[0026] In the preferred embodiments of the present invention, aquaternary ammonium compound such as a choline-derivative compound isadded to the slurry in which polishing particles are suspended indeionized water, so that the removal selectivity relative to the siliconnitride layer can be effectively increased. FIG. 2 illustrates theeffects of adding the choline chloride of FIG. 1 to the slurry.

[0027] To obtain the illustrated measurements, a PRESl polisher was usedas a chemical mechanical polishing apparatus. A down force of 6 psi anda platen rotating speed of 65 rpm were used. A slurry manufactured bydiluting slurry used for the oxide with deionized water was used. Inorder to exclude any influence of the pH on the removal rate, the pH ofthe slurry was maintained at 11.2. For the CMP, the silicon nitridelayer and the silicon oxide layer are respectively formed on the surfaceof the respective wafer.

[0028] For comparison, the removal rate of the silicon oxide layer(PE-TEOS) obtained under the above conditions in the absence of thecholine chloride was 2501 Å/min, and the removal rate of the oxidenitride layer was 588 Å/min. The removal selectivity of the siliconoxide layer to the silicon nitride layer, calculated from these values,was 4.25. The pH of the entire slurry was maintained at 11.2 forcomparison under the same conditions.

[0029] As shown in FIG. 2, in the case where the choline chloride ofFIG. 1 was added to the slurry as according to the preferred embodimentof the present invention, the removal selectivity becomes almost threetimes higher than that of the case where the choline chloride was notadded. Therefore, these results demonstrate that the removal selectivityof SiO₂:Si₃N₄ can be effectively improved by the preferred embodimentsof the present invention, i.e, by adding the describedcholine-derivative compounds such as [(CH₃)₃NCH₂CH₂OH]Cl,[(CH₃)₃NCH₂CH₂OH]l, [(CH₃)₃NCH₂CH₂OH]Br, or [(CH₃)₃NCH₂CH₂OH]CO₃.

[0030] The improved removal selectivity resulting from the addition ofthe choline-derivative compound to the slurry, may be due to the actionof the choline cation which is dissociated in the choline-derivativecompound, for example, [(CH₃)₃NCH₂CH₂OH]+. However, if too large anamount of the choline-derivative compounds is added, as is apparent fromFIG. 2, the removal rate of both the silicon nitride layer and thesilicon oxide layer can decrease substantially.

[0031] Thus, the amount of added choline-derivative compound which canimprove the removal selectivity of the silicon oxide layer on thesilicon nitride layer is considered to be about 0.1-5.0 wt% of theslurry. From the results of the graph shown in FIG. 2, it is preferableto add the choline chloride in an amount of about 0.5-3.0 wt% of theslurry.

[0032] If the amount of the choline cation which is formed bydissociating the choline-derivative compound in the slurry is set as astandard, it is preferable to add the choline-derivative compound, forexample choline chloride, to the slurry in order to make the amount ofthe choline cation in the slurry about 0.001-5 M, preferably 0.1-1 M.

[0033] The effect of adding the choline-derivative compound can beembodied by adding the single choline compound, for example cholinechloride, as shown in FIG. 2. However, two or more kinds ofcholine-derivative compounds can be mixed, and the removal selectivitycan be effectively improved by adding the mixture to the slurry.

EXAMPLE 2

[0034] If the pH of the slurry changes when the choline-derivativecompound is added to the slurry, the removal rate of the silicon oxidelayer and the silicon nitride layer can change, which in turn can changethe removal selectivity. Table 1 shows changes in the removal rate andremoval selectivity caused by changing the pH of the slurry, fixing theamount of added choline chloride, and using sulfuric acid (H₂SO₄) andkalium hydroxide (KOH). The amount of added choline chloride isarbitrarily fixed at 1.39 wt% in which the choline cation has a molarconcentration (M) of 0.1 in an aqueous solution. TABLE 1 pH PE-TEOS(Å/min) Si₃N₄ (Å/min) Removal selectivity 11.2 1736 177 9.81 12.45 2912433 6.73 12.9 3089 407 7.59

[0035] As shown in Table 1, variation of the pH can alter the removalrate and the removal selectivity of the silicon oxide layer and thesilicon nitride layer. In particular, when the pH increases, the removalrate of the silicon oxide layer increases. This is because the hydrationrate of the silicon oxide layer increases when there is a high pH, sothat the removal rate of the silicon oxide layer can increase.Therefore, the increase in the hydration rate can compensate for thedecrease in the removal rate of the silicon oxide layer resulting fromthe addition of the choline chloride.

[0036] As suggested in Table 1,when the choline chloride is added, it ispreferable that the slurry has a pH of about 9-13.5. In order toincrease the removal rate of the silicon oxide layer, it is preferablethat the slurry has a pH of about 12.

[0037] The control of the pH of the slurry can be accomplished by addingextra pH control agent. The choline-halogen compounds suggested in thepresent invention such as choline chloride do not substantially changethe overall pH even though they are added to the slurry and dissociatedin aqueous solution. Therefore, it is preferable to control the pH byadding an extra pH control agent.

[0038] A base or an acid can be used as the pH control agent. Basesinclude kalium hydroxide (KOH), ammonium hydroxide (NH₄OH) ortetramethyl ammonium hydroxide (TMAH: (CH₃)₄NOH). Also, cholinehydroxide ([(CH₃)₃NCH₂CH₂OH]OH) can be used as the base. Acids includehydrochloric acid (HCI), sulfuric acid (H₂SO₄), phosphoric acid (H₃PO₄)or nitric acid (HNO₃). The pH of the slurry can be maintained at acertain level by adding the acid and the base to the slurry.

[0039] Even though the pH of the slurry is maintained to be constant,the removal selectivity of the silicon oxide layer on the siliconnitride layer can change according to the kind of pH control agent used.This is illustrated in the graph of FIG. 3. In particular, FIG. 3 showsthe results of measuring the removal selectivity upon adding KOH, TMAHand choline hydroxide, respectively, as the base among the pH controlagents to the slurry including choline chloride. Here, the pH of theslurry is set to be 12.5, and the sulfuric acid is used as the acid.

[0040] According to the results illustrated in FIG. 3, the differencebetween the removal selectivity when using TMAH and that when using KOHis not so large. However, when using KOH, the removal selectivity of theindividual layer materials increases compared to the case of using TMAH.When using the choline hydroxide as the base, however, a relatively highremoval selectivity can be obtained.

EXAMPLE 3

[0041] The amount of pH control agent, that is, choline hydroxide, addedto the slurry can impact the removal selectivity.

[0042] Table 2 shows the change in removal selectivity according to theamount of choline hydroxide added as the base among the pH controlagents. In particular, the change of the removal selectivity withrespect to different choline chloride to choline hydroxide ratios, wasmeasured, and is illustrated in Table 2. To make the total amount ofcholine cations in the slurry regular, that is, to fix the concentrationof choline cations at 0.1 M, the relative amounts of the cholinechloride and the choline hydroxide are changed. TABLE 2 Mol Rate ofCholine Chloride:Choline Removal Hydroxide PE-TEOS (Å/min) Si₃N₄ (Å/min)selectivity 1.0:0.0 1736 177 9.81 0.75:0.25 1860 164 11.3 0.5:0.5 1457104 14.0 0.25:0.75 1055 72 14.7

[0043] The result of Table 2 shows that when the amount of added cholinehydroxide increases, although the overall amount of choline cationswithin the slurry is fixed, the removal selectivity can be increased.The increase in the amount of added choline hydroxide causes an increasein the relative amount of OH⁻ions within the slurry, so that the pH ofthe slurry increases. That is, as the pH increases, the removalselectivity also increases.

[0044] When the choline hydroxide is added to the slurry including thecholine-derivative compounds as the pH control agent, the amount ofadded choline-halogen compound is, preferably, controlled to make theconcentration of the entire choline cations within the slurry about0.001-5 M, more preferably, 0.1-1 M.

EXAMPLE 4

[0045] As suggested in a preferred embodiment of the present invention,the removal selectivity of the silicon oxide layer on the siliconnitride layer can be improved by adding a surfactant to the slurry. Thisimprovement effect can be demonstrated by the results of Table 3 whichshows the changes in the polishing removal selectivity according to theaddition of the polyethylene oxide. In particular, the results of adding1.3% by weight choline chloride and 0.05% by weight polyethylene oxide(PEO) as the surfactant are tabulated in Table 3. The pH of the slurryis maintained at 11.2. TABLE 3 Amount of Choline Chloride:CholineRemoval Hydroxide PE-TEOS (Å/min) Si₃N₄ (Å/min) selectivity 1.3 wt %:0.0 wt % 2070 399 5.2 1.3 wt %: 0.05 wt % 2288 283 8.1

[0046] As suggested in Table 3, the removal selectivity of the siliconoxide layer on the silicon nitride layer can be improved by adding thePEO with the choline chloride to the slurry. A low molecular surfactantor a polymer surfactant can be used as the surfactant. The surfactantincludes a cation surfactant such as cetyidimethyl ammonium bromide orcetyltrimethyl ammonium bromide, and a deionized surfactant such as awell-known brand name, “Brij^(™)”, or “Tween^(™)”. Also, a highmolecular surfactant such as a well-known brand name “Pluronic^(™)”,PEO, polyvinyl alcohol (PVA) or polyethylene glycol (PEG) can beincluded. Preferably, the amount of surfactant added to the slurry isabout 0.001-1 wt% based on the weight of the slurry.

[0047] According to the preferred embodiments of the present invention,the slurry can be used in polishing a wafer surface having a siliconoxide layer and silicon nitride layer. Here, the silicon oxide layer canbe formed to fill an STI structure, and the silicon oxide layer can bepatterned by polishing the silicon oxide layer using the silicon nitridelayer formed in the lower portion of the silicon oxide layer as apolishing stop layer.

[0048] The removal selectivity of the silicon oxide layer on the siliconnitride layer can be improved by performing the CMP process using theslurry according to the preferred embodiments of the present invention.Therefore, it becomes easier to control the CMP process, and the processmargin can be enhanced.

[0049] The present invention has been described in detail through thepreferred embodiments thereof. However, the present invention may bechanged or improved by those skilled in the art without departing fromthe spirit and scope of the invention.

What is claimed is:
 1. A wafer polishing slurry, comprising: a pluralityof polishing particles; a solvent in which the polishing particles aresuspended; and a quaternary ammonium compound contained in the solventand having a form of {N-(R₁R₂R₃R₄)}⁺X⁻, in which R₁, R₂, R₃ and R₄ areradicals and X⁻is an anion derivative.
 2. The wafer polishing slurry ofclaim 1, wherein the plurality of polishing particles are selected fromthe group consisting of silica, alumina, titania, zirconia, germania,ceria, and mixtures thereof.
 3. The wafer polishing slurry of claim 1,wherein an amount of the polishing particles is about 1-30 wt% based onthe weight of the slurry.
 4. The wafer polishing slurry of claim 1,wherein the solvent comprises deionized water.
 5. The wafer polishingslurry of claim 1, wherein each of R₁, R₂, R₃, and R₄ of the quaternaryammonium compound are an alkyl group or an alkanol group.
 6. The waferpolishing slurry of claim 5, wherein each of R₁, R₂, and R₃ of thequaternary ammonium compound are a methyl group, and R₄ of thequaternary ammonium compound is an ethanol group.
 7. The wafer polishingslurry of claim 1, wherein the quaternary ammonium compound comprises[(CH₃)₃NCH₂CH₂OH]Cl, [(CH₃)₃NCH₂CH₂OH]l, [(CH₃)₃NCH₂CH₂OH]Br,[(CH₃)₃NCH₂CH₂OH]CO₃, or mixtures thereof.
 8. The wafer polishing slurryof claim 1, wherein the quaternary ammonium compound comprises[(CH₃)₃NCH₂CH₂OH]Cl.
 9. The wafer polishing slurry of claim 1, whereinan amount of quaternary ammonium compound is about 0.1-5 wt% based onthe weight of the slurry.
 10. The wafer polishing slurry of claim 1,wherein an amount of quaternary ammonium compound is about 0.5-3 wt%based on the weight of the slurry.
 11. The wafer polishing slurry ofclaim 1, wherein the quaternary ammonium compound is dissociated in thesolvent, and provides a {N-(R₁R₂R₃R₄)} cation concentration of about0.001-5 M.
 12. The wafer polishing slurry of claim 1, wherein thequaternary ammonium compound is dissociated in the solvent, and providesa {N-(R₁R₂R₃R₄)}⁺cation concentration of about 0.01-3 M.
 13. The waferpolishing slurry of claim 1, wherein the slurry has a pH value of9.0-13.5.
 14. The wafer polishing slurry of claim 1, further comprisinga pH control agent.
 15. The wafer polishing slurry of claim 14, whereinthe pH control agent comprises a base selected from the group consistingof KOH, NH₄OH, and (CH₃)₄NOH; and an acid selected from the groupconsisting of HCI, H₂SO₄, H₃PO₄, and HNO₃.
 16. The wafer polishingslurry of claim 14, wherein the pH control agent comprises[(CH₃)₃NCH₂CH₂OH]OH.
 17. The wafer polishing slurry of claim 14, furthercomprising a surfactant.
 18. The wafer polishing slurry of claim 17,wherein the surfactant is cetyldimethyl ammonium bromide, cetyldimethylammonium bromide, polyethylene oxide, polyethylene alcohol orpolyethylene glycol.
 19. The wafer polishing slurry of claim 17, whereinan amount of surfactant is 0.001-1 wt% based on the weight of theslurry.
 20. A wafer polishing slurry, comprising: a plurality ofpolishing particles; a solvent in which the polishing particles aresuspended; a quaternary ammonium compound contained in the solvent andhaving a form of {N-(R₁R₂R₃R₄)}⁺X⁻, in which R₁, R₂, R₃, and R₄ areradicals, and the X⁻is an anion derivative; and a surfactant which isadded to the solution.
 21. The wafer polishing slurry of claim 20,wherein the quaternary ammonium compound comprises [(CH₃)₃NCH₂CH₂OH]Cl,[(CH₃)₃NCH₂CH₂OH]l, [(CH₃)₃NCH₂CH₂OH]Br, [(CH₃)₃NCH₂CH₂OH]CO₃, ormixtures thereof.
 22. The wafer polishing slurry of claim 21, whereinthe surfactant is cetyldimethyl ammonium bromide, cetyidimethyl ammoniumbromide, polyethylene oxide, polyethylene alcohol or polyethyleneglycol.
 23. A wafer polishing method comprising: providing asemiconductor wafer having a silicon oxide layer and a silicon nitridestop layer; and chemical mechanical polishing the silicon oxide layerand the silicon nitride stop layer using a polishing slurry whichcomprises an additive of a quaternary ammonium compound having a form of{N-(R₁R₂R₃R₄)}⁺X⁻, in which R₁, R₂, R₃, and R₄ are radicals, and X⁻is ananion derivative.
 24. The wafer polishing method of claim 23, whereinthe quaternary ammonium compound comprises [(CH₃)₃NCH₂CH₂OH]Cl,[(CH₃)₃NCH₂CH₂OH]l, [(CH₃)₃NCH₂CH₂OH]Br, [(CH₃)₃NCH₂CH₂OH]CO₃, ormixtures thereof.
 25. The method of claim 23, wherein the slurry furthercomprises a surfactant.
 26. The method of claim 25, wherein thesurfactant is cetyldimethyl ammonium bromide, cetyidimethyl ammoniumbromide, polyethylene oxide, polyethylene alcohol or polyethyleneglycol.